There's More Than One Way to Skin a Block – Tips on Unstructured Block Assembly Using Assemble Special

There are two ways to construct blocks in Pointwise, automatically or
manually. The question is, if automatic block assembly exists, when is
it necessary to use the manual block assembly tools?

To understand Pointwise block assembly, an explanation of its foundation
is helpful. Pointwise uses the notion of a “face” to define
how blocks
are assembled. A structured block is defined by six faces. An
unstructured block is defined by at least one outer face and possibly
many more interior faces. Furthermore, it is important to understand
that each face can contain multiple domains.

For structured blocks, manual assembly (what Pointwise calls Assemble
Special) will be required when the topology of the block is too complex
for the automatic assembly to determine the correct connections between
the faces. This can occur when there are multiple domains per face, and
requires the use of Assemble Special so the user can define each of the
six block faces individually. An indication that the block topology is
too complex for automatic assembly is when the Assemble Blocks function
on the toolbar simply fails to create the desired block.

It is easier to decide whether to use automatic or manual assembly for
unstructured blocks than it is for structured blocks. If an unstructured
block consists of a single closed face, the automatic assembly will be
the logical choice. Just select the domains of this face, assuming they
are watertight, and click Assemble Blocks from the toolbar and Pointwise
will create an unstructured block. If the block includes more than one
face, it must be assembled manually using Assemble Special.

Why Use Interior Faces?

Additional faces in an unstructured block can serve multiple purposes.
The addition of internal, closed faces can be used to designate the
boundaries of a physical object, like an insect (Figure 1), within an
unstructured block.

Figure 1: Generic flapping wing insect grid. Insect geometry courtesy of Alok Rege, a graduate student in the Department of Mechanical and Aerospace Engineering at the University of Texas at Arlington.

Interior faces can also serve as an interface between blocks, allowing
structured, unstructured, or hybrid blocks to be embedded within the
primary flow domain. Such embedded blocks allow for explicit grid point
distribution control in critical regions, such as the large wake region
behind a vehicle, shown in Figure 2.

Figure 2: Sedan wake region.

Non-manifold interior faces, called baffles, can also be used to add
internal topology to an unstructured block. Simply put, a baffle is a
non-closed face embedded within an unstructured block. A baffle can be
free floating or connected to other baffles and closed faces. Similar to
internal, closed faces, baffles can be used for explicit grid point
control and can even be used to represent thin walls, like the wings of
an insect, shown in Figure 3.

Figure 3: Use baffles to represent thin walls, like the wings of an insect.

Now that we know when to use the manual block assembly tools, let's take
a look at the process for creating an unstructured block with multiple
faces in Pointwise.

How to Assemble an Unstructured Block with Multiple Faces

Manual block construction is accomplished through the Pointwise menu
command Create, Assemble Special, Block (Figure 4). This opens a panel
is where each face of the block is defined and saved, resulting in the
final definition of the unstructured block. Assemble Special cannot be
used to add faces to an existing block. That is done using the Add Faces
command, which will be explained later.

The first step when creating an unstructured block consisting of
multiple faces is to select the domains defining the outer boundary of
the volume, in this example, the farfield domains. Once one of the
farfield domains has been selected, remaining adjacent domains will be
highlighted in green, shown in Figure 5. Only adjacent domains can be
selected to close the face. For faces that consist of multiple domains,
select the first domain, then select the remaining domains by using the
All Adjacent selection tool in the Select menu, or use the keyboard
shortcut Ctrl+Shift+A.

Figure 5: Adjacent domains are highlighted in green.

With all of the farfield domains selected, click Save Face to save the
first face of the block. Notice in Figure 6 the orientation of the first
face is pointed into the volume. If the face orientation was not
correct, it could be flipped by clicking Begin Flip Face Orientation or
the face could be removed by selecting Remove Last Face.

Figure 6: Farfield block orientation.

When you are satisfied with the first face of the block, additional
faces can be constructed. Here, the domains on the body of the insect
represent a set of closed faces within the unstructured farfield block.
For the simulation, the region inside the insect is a void and does not
need to be discretized. By selecting the insect domains as the second
(head) and third (thorax + abdomen) faces of the block, a Boolean
subtraction takes place that effectively removes the region defined by
the insect from the unstructured block. As a result, the orientation of
these faces should be pointing outward into the volume.

The wing domains are to be modeled as thin surfaces. As non-manifold
surfaces, the wing domains are selected as the fourth and fifth faces
and added to the block using the Save Baffle Face command (Figure 7).
You will notice face orientation arrows emanating from both sides of the
baffle faces. The block can then be saved and initialized.

Figure 7: Unstructured block consisting of multiple faces.

Adding a Face to an Existing Block

If you discover you need more precise control of the grid, for example,
in the downstream wake of the insect, it is possible to add a baffle
face without going through the block assembly process again. You can
add faces to an existing block using the Add Face command from the Edit
menu (Figure 8). In fact, if a block was constructed from only the outer
domains, all interior faces could be added in this manner. One thing to
keep in mind is that the addition of a face to a preexisting block
causes the block interior to be emptied, thus requiring that the block
be reinitialized.

The result of adding a baffle face in the wake can be seen in Figure 9.
The baffle domain fixes points within the volume, providing local grid
point control precisely where it is desired. In this case, the baffle is
used to help better resolve unsteady flow phenomena downstream of the
insect.

Figure 9: Using a baffle for local grid point control.

In this example, the body of the insect was defined by two closed faces,
whereas the wings and wake sheet were added as baffle faces. Remember,
baffles can be used either to control grid point spacing and/or act as
infinitely thin surfaces. Topologically these two types of baffles are
the same, so where is the distinction made? It is defined in the CAE,
Set Boundary Conditions panel, shown in Figure 10. Both sides of a
baffle become selectable once the Select Connections checkbox is
enabled. Therefore, the two sides of each wing can be placed in a single
boundary condition with the CAE type set to wall, whereas the wake sheet
domain retains the Connection designation, and is used only for
resolution control.

Figure 10: Boundary conditions panel in the CAE menu.

Summing It Up

You should now have a better understanding of the manual assembly
procedure for unstructured blocks. To summarize:

If your unstructured block is composed of multiple faces, use the
Create, Assemble Special, Block command.

Adding faces to an existing block can be accomplished using the menu
command Edit, Add Face.

Internal, closed faces can either mark the surfaces of an object or
interface between blocks.

Baffles can be added to an unstructured block for resolution control
or modeling of thin surfaces.

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